1. Field of the Invention
This invention relates to motor control. More specifically, the invention is a control system and method for use with position-controlled motors that allows the motor to be stopped precisely without overshoot even when the motor is operating at or near full speed.
2. Description of the Related Art
Various motors and their control systems are known in the art. One conventional motor/control system is illustrated in block diagram form in FIG. 1 where a position loop is closed around a motor 10. More specifically, motor 10 incorporates or is coupled to a drive mechanism 12 that typically rotates or moves linearly. Drive mechanism 12 (and/or motor 10) has a position sensor 16 coupled thereto to provide signals indicative of actual position of drive mechanism 12 at any given time for use as a feedback input to a motor controller 18. A commanded position is issued from a master control (not shown) to motor controller 18 which uses the command in conjunction with the position feedback signals to control motor 10.
Full speed of motor 10 is reached when the motor's inertia and drive mechanism's inertia are both accelerated to full speed for a given motor voltage. How quickly motor 10 can accelerate is usually limited by the maximum current to motor 10. For applications where the maximum speed of the drive mechanism is desired, motor 10 is run at it's full speed when provided with a certain supply voltage. If the inertia of motor 10 and drive mechanism 12 is low, motor 10 (and drive mechanism 12) can be stopped using just a standard position control loop. However, for systems with greater inertia or systems requiring a greater degree of motor damping, rate or speed feedback can be used in conjunction with actual position feedback. That is, it is well known in the art of control engineering to improve or stabilize a position control loop by adding in rate or speed feedback. This is illustrated in FIG. 1 where a speed sensor 14 is coupled to drive mechanism 12 (and/or motor 10) to provide motor controller 18 with the actual speed of drive mechanism 12. Specifically, as is known in the art of motor control, motor controller 18 uses position information from sensor 16 to determine when to stop motor 10 and uses speed information from sensor 14 to generate a damping coefficient.
However, there is one problem associated with this type of system when motor 10 runs at or near full speed while moving drive mechanism 12 from a first position to a second position. Once the second position has been reached, little or no overshoot is desired. Yet, this can be very difficult to achieve, especially in high inertia systems. If the speed feedback signal is used to obtain the desired damping coefficient during the motor's entire run cycle, motor 10 will not run at full speed as drive mechanism 12 moves from the first to second position. On the other hand, if no speed feedback is used and motor 10 is allowed to run at or near full speed as drive mechanism 12 travels from the first to second position, the desired damping coefficient will not be available when drive mechanism 12 reaches the second position.